1. Field of the Invention
The present invention relates generally to an image generating apparatus and an image generating program product employing an electrophotographic method to generate an image by performing area coverage modulation to express halftone colors.
2. Description of the Related Art
In order to display or print out a monochrome gradation image with a display screen or a printer that can only display or print out two (binary) colors, i.e., black and white, the area coverage modulation method is employed. Specifically, the ratio of areas of black and white is varied to reproduce gradations (see, for example, Patent Document 1). Accordingly, the density pattern method is typically employed, which involves dividing each of the pixels in image data into a matrix of n×n. Each of the n×n sub pixels obtained by dividing the pixel (hereinafter, “sub pixels”) is configured to display a black dot. To reproduce gradations, the number of black dots displayed by sub pixels in the matrix is varied according to the intended gradation level. In the conventional technology, when an image is generated by the area coverage modulation method, it is assumed that each sub pixel is a square, and gradations of the image are calculated simply based on the number of black dots displayed by sub pixels.
There are image generating apparatuses such as printers and copiers that can produce an image with hidden characters embedded therein. The hidden characters are hidden in the original but appear in a copy of the original, so that the copy can be distinguished from the original. There is a technology of treating the original image and the hidden characters by different dithering processes, so that an image is intentionally made unrecognizable (see, for example, Patent Documents 2, 3).
In the invention described in Patent Document 2, an image is output based on image data including dots of a first line number and hidden character data including dots of a second line number. In an original document, the hidden characters cannot be recognized. However, in a copy of the original document, the hidden characters clearly appear due to properties of the image generating apparatus used for making the copy.
In the invention described in Patent Document 3, dither patterns are generated for different line numbers and a density balance confirmation pattern is output. When it is determined that there are irregularities in the sampled density, a more appropriate dither pattern is selected to correct such irregularities. Accordingly, even if density levels differ among different apparatuses, it is possible to eliminate failures in which hidden characters do not appear.
Patent Document 1: Japanese Laid-Open Patent Application No. 2001-086358 (page 2)
Patent Document 2: Japanese Laid-Open Patent Application No. H7-231384 (FIG. 5)
Patent Document 3: Japanese Laid-Open Patent Application No. 2005-260553 (FIG. 2)
However, when images are generated by the area coverage modulation, patches (irregularities) may appear in the gradation. FIG. 7 illustrates dither patterns 71, 72 of a pixel. The pixel is divided by 4×4=16 to form a matrix including 16 sub pixels. When none of the sub pixels are displaying black dots (0/16), the pixel displays white. When all of the sub pixels obtained by dividing the pixel are displaying black dots (16/16), the pixel displays black. The dither patterns 71, 72 both have four sub pixels displaying black dots. In the conventional technology, both of the dither patterns 71, 72 are used for expressing the same density level of 4/16. However, the actual output image using the dither pattern 71 and the actual output image using the dither pattern 72 appear to have different densities. This is probably because dither patterns express density levels based on the number of square sub pixels displaying black dots, which is inappropriate for expressing the actual toner density. That is, even if different dither patterns include the same number of sub pixels displaying black dots, the actual density may differ according to the positional arrangement of the black dots.
One approach is to use a matrix 80 as shown in FIG. 8 for expressing gradations. Each of the sub pixels configured to display a black dot in the matrix 80 is denoted with a number. These numbers represent the order in which the sub pixels display black dots according to the density level. To display an appropriate number of black dots corresponding to an intended density level, the black dots are displayed at sub pixels (positions) defined by the numbers. For example, to display a density level of 5/16, five black dots need to be displayed. Therefore, the five dots are displayed at the sub pixels denoted by “1”-“5” in the matrix 80. In this manner, by using the matrix 80, the positions of the black dots can be defined. As a result, it is possible to prevent spots (irregularities) from appearing in the gradations.
However, even by using the matrix 80, the density in an actual printout and the density in image data appear to be different. Furthermore, when the matrix 80 is applied for producing documents with hidden characters as described above, there are cases where the densities become inconsistent and the characters are not completely hidden.